The receiving circuit of a mobile radio apparatus normally includes a so-called equalizer which is operative to compensate for irregularities or deficiencies in the radio medium when transmitting radio signals from a base station. An equalizer is primarily used in receiving circuits for the purpose of reducing of multipath propagation in the radio medium. This is described, for instance, in WO 88/05981, which relates to a TDMA-system which includes so-called adaptive equalization. The setting of the equalizer incorporated in the radio receiver is contingent on synchronizing words that are sent simultaneously with the data words transmitted from the radio transmitter in time multiplex. With the aid of these synchronizing words, the equalizer can be set so as to equalize the dispersion properties of the medium upon receipt of the data words. Radio receivers which include equalizers are primarily used for high symbol rate communication (&gt;100 kbit/s) the bit sensitivity of which due to multipath propagation is greater than the bit sensitivity of slower symbol rate communication.
The need for an equalizer is also determined by the nature of the surroundings, in addition to the symbol rate. The lowest multipath propagation effect is obtained, for instance, from a totally level landscape which is devoid of buildings.
The advantage of an equalizer is that it is able to equalize time dispersion in the signals received, provided that coherent detection is applied. The disadvantage of the equalizer is that it is relatively complex and has a relatively high power consumption.
The absence of an equalizer affords the advantage of enabling noncoherent demodulation to be applied, which results in a lower degree of complexity in the receiver and a lower current consumption. In addition, a robust receiver is obtained with rapidly varying radio channels, due to high vehicle speeds. The disadvantage lies in the fact that the demodulation cannot be carried out with time dispersion, which constitute a considerable part of the symbol time.
It is previously known in Swedish patent application No. 8902844-3 to connect or to disconnect the equalizer in the mobile station during a call in progression depending on the estimated time dispersion.